Raw coal, as it is taken from a mine, consists of lumps and particles of coal which differ both in size and degree of purity. Before such coal may be shipped to a consumer it must be crushed and sized and must meet specified standards of purity in order that it be fit both for shipping and for its intended use. Specifically, the coal must often be separated both from refuse, i.e. pyrites, slate, clay, etc., and from water, and other liquids, used to separate such refuse.
To effect such separation, the raw coal is frequently subjected to a crushing operation and then to a series of screening operations which pass the oversizes, i.e. the coal that does not pass through the screens, to jigs, heavy medium cyclones or hydrocyclones or other appropriate apparatus where refuse is removed.
The coal particles which pass through all screening operations, i.e. the coal undersizes, are designated "fines" and are typically on the order of minus sixty-five mesh. These fines represent a significant percentage of the coal mined and, therefore, the overall economics of the mining operation are dependent upon an efficient separation and recovery of these fines from their impurities.
Conventional methods for the purification and recovery of fines comprise routing an aqueous slurry of these fines to a flotation cell in which the slurry is treated with an organic reagent such as, for example, methyl isobutyl carbinol or 2-ethylisohexanol. These reagents, by virtue of their affinity for carbonaceous surfaces, coat the coal particles, while leaving the non-carbonaceous particles of refuse unchanged. The flotation bath in which such coating is effected is vigorously agitated by conventional stirring means and by contrant aeration. As a result of this agitation and aeration, the alcohol-coated coal fines tend to adhere to the air bubbles and rise to the surface of the bath, forming a so-called flotation froth. At the same time, the uncoated refuse particles tend to remain in suspension in the flotation bath and are withdrawn therefrom and discarded. With sixty-five mesh coal, the flotation froth may typically contain perhaps 15% coal particles and as much as 20% of coal particles by weight, the exact percentage being dependent on particle size, shape and density, type of flotation machine and manner of operation.
The flotation froth is continuously removed from the surface of the bath and vacuum filtered in order to dewater the coal. The fine coal is then removed from the filter and either used as is or recombined with the larger coal sizes separated by prior screening and processing steps.
The use of these conventional techniques results in acceptable purification of the fine coal in the flotation step itself. However, the filtration step has been less than satisfactory due to at least two limiting factors directly resulting from the sheer amount of water contained in the flotation froth.
The first limiting factor arises from the fact that virtually all filters are limited in the gross volume of filterable solution they can handle per unit time by the size of the filter. Thus one consequence of the high water to coal weight ratio in conventional flotation froths is that the sheer volume of froth containing a given amount of water may exceed the filter's physical capacity or volume and thereby limit the hourly volume of froth which can be filtered. In other words, although a given filter may be capable of handling the amount of water contained in a given amount of flotation froth, the sheer volume of froth containing water may be too great for the filter to handle efficiently, or even at all. As a result, the overall rate of coal processing may be seriously limited by the filtration step.
The second limiting factor in the conventional process is the low yields of coal per unit time. This low isolation rate (expressed, for example, in pounds of coal isolated per hour) is directly attributable to the relatively low percentage of coal present in any given volume of conventional flotation froth.
It is important in many filters, furthermore, particularly in drum or disk type vacuum filters, for the medium being filtered to contain sufficient solids to initially coat the filter element in order to establish a differential pressure between the downstream side of the filter and the upstream side. If, for example, a drum or disk type filter is presented with a froth containing only a little particulate material such as fine coal, the surface of the filter in the absence of a coating of particulate will remain so porous that insufficient pressure difference between one side of the filter medium and the other will be established to conduct any substantial filtering at all. Thus in those cases in which the froth volume compared to the solids content of the froth is very large initiation of filtering may be delayed for a finite period as the solids build up on the filter surface thus delaying the initiation of efficient filtering and possibly allowing more fine solids than usual to pass through the filtering medium or filter cloth.
In order to avoid these difficulties a coarser coal product from a previous separation step has often been combined with the froth, or flotation product, prior to filtering to increase the percentage of coal in the flotation product and increase the efficiency of filtering. The combination of the prior coarser coal, for example, coal having a particle size greater than 65 mesh, with the flotation coal product of 65 mesh or less naturally provides a mixed coal size which may, or may not, be desirable depending upon circumstances. A more serious problem arises, however, when the flotation product must be thermally dried after filtering, but prior to use. Fine coal particles of about 65 mesh or less have a large aggregate surface area and tend to retain a large amount of moisture. A fairly large amount of fuel is necessary to generate the heat values to remove this moisture. Under these circumstances it is inefficient to mix a coarser coal product with the fine coal flotation product and thermally dry the mixed coal product. The coarse coal does not normally retain sufficient moisture to require thermal drying and thus adds a significant amount of bulk to the mixed product, all of which must be heated, without adding moisture which requires removal. The efficiency of the drying operation is thus substantially decreased.
It has been customary under somewhat similar circumstances in the filtering of mineral flotation products, for example, mineral ores and the like, to thicken the flotation product by the use of a thickener or clarifier. In these systems the flotation product is conveyed to a thickener where it is retained on the surface of the liquid body in the thickener until the froth breaks down and the mineral particles settle to the bottom of the thickener. The underflow is then conveyed to a suitable filter. Sometimes sprays are used to accelerate decomposition of the flotation foam or froth on the surface of the thickener.
While flotation followed by thickening in a thickener, or settling apparatus, has been successfully used in the treatment of mineral ores, it has not proven satisfactory in the treatment of coal. The mineral particles in an ore flotation froth product have a higher specific gravity than coal particles and are less resistant to wetting than coal froth solids. Consequently, while mineral particles are readily released from a flotation froth, coal solids in a flotation froth product are not readily released from the froth in a conventional thickener even with auxiliary spraying. The froth persists and only a portion of the fine coal product will settle to the bottom of the thickener in a condition for filtering. In the case of coal, once the coal has been floated it is almost totally impossible without chemical treatment to sufficiently wet all the floated coal particles to release the coal from the froth so that a conventional thickener can be used to prepare the flotation product for efficient filtering. Some amount of coal will always remain attached to the air bubbles and remain on the surface of the thickening vessel.
It would thus be desirable to decrease the volume of coal flotation froth product prior to filtration. It would, furthermore, be desirable to decrease such volume while increasing the percentage of coal particles per unit volume of froth product in order that fine coals may be more rapidly isolated therefrom.